In high temperature gasification processes, a hot partial oxidation gas is produced from hydrocarbonaceous fuels, for example coal. In these processes, the hydrocarbonaceous fuels are reacted with a reactive oxygen-containing gas, such as air or oxygen, in a gasification reactor to obtain the hot partial oxidation gas.
In a typical gasification process, the hot partial oxidation gas will substantially comprise H2, CO, and at least one gas from the group H2O, CO2, H2S, COS, NH3, N2, Ar, along with particulate carbon, ash, and/or molten slag typically containing species such as SiO2, Al2O3, and the oxides and oxysulfides of metals such as Fe and Ca.
The hot partial oxidation gas in the gasification reactor will commonly be at a temperature ranging from 1,700° to 3,000° F., and more typically in the range of about 2,000° to 2,800° F., and at a pressure commonly in the range of about 1 to about 250 atmospheres, and more typically in the range of about 15 to 150 atmospheres.
Thermocouples are commonly used for measuring temperature in these high temperature processes. The thermocouples can be used to measure the temperature in the gasification reactor. They may also be used to measure the temperature in downstream process steps in which the effluent is cooled and particulate and gaseous contaminants are removed.
Thermocouples are pairs of wires of dissimilar metals which are connected at both ends. The content of the wires must be sufficiently dissimilar to allow for a difference in electrical potential between them. Except for the ends, the two wires are electrically insulated from each other. The electrical insulation is commonly provided by a tube of insulating material having two non-intersecting holes passing lengthwise through the tube. Typical insulating materials include high temperature, high purity ceramics, such as alumina.
When the two junctions of the wires are at different temperatures, a difference in electrical potential exists between them. The difference in electrical potential and therefore the difference in temperature can be measured by a voltage measuring instrument placed in the thermocouple circuit or alternatively by a voltage measuring instrument that is sent signals by a transmitter placed in the thermocouple circuit.
The choice of dissimilar metals used for the thermocouple will vary depending on, among other things, the expected temperature range to be measured. For instance, one type of thermocouple commonly employed under the conditions present in a gasification reactor has one wire that contains platinum and about 30% rhodium and a second wire that contains platinum and about 6% rhodium. Other pairs of metals are used for different temperature ranges.
One problem apparent with the use of thermocouples in the environment present in a gasification process, particularly the environment present in the gasification reactor, is the relatively short lifespan of the thermocouples. The relatively short lifespan is due in part to the extremely high temperatures and corrosive atmosphere that prevails during the operation of the gasification reactor. An unprotected thermocouple left in this environment is quickly attacked and rendered useless. Such attack can be most severe when the thermocouple comes into contact with molten slag present in the reactor.
To alleviate this problem, thermocouples are commonly inserted into a refractory thermowell disposed along the outer wall of a gasification reactor or other exterior process surface. The refractory thermowells would include barriers of chrome-magnesia, high chrome, or similar slag resistant materials, and may incorporate other refractory and non-refractory materials such as Al2O3, MgO, and stainless steel.
When used in a gasification reactor, the thermowell may be introduced by passing it through an opening in the outer wall of the reactor pressure vessel. The thermowell may then pass through a corresponding opening in a refractory material, or series of refractory materials, commonly used to line the inner surface of the reactor pressure vessel. The thermowell may extend into the open space of the reactor or it may be set is back at a slight distance from the interior of the reactor.
Unfortunately, positioning the thermocouple inside a thermowell has not provided a complete solution. Over time, molten slag will breach the thermowell. The breach is commonly due to the effects of erosion and corrosion as well as thermal and/or mechanical stress. However, the breach may also be due, totally or in part, to an inherent fault in the thermowell. The breach, typically small initially, allows molten slag to enter the thermowell where it can come in contact with the thermocouple, rendering it useless.
It would therefore be beneficial to have a means to increase the lifespan of thermocouples used in a gasification process.
In accordance with one aspect of the present invention, an improved apparatus comprising a thermocouple for measuring the temperature in a gasification process is provided. The improvement comprises a sapphire envelope for enclosing at least a portion of the thermocouple. The sapphire envelope may be in the form of a sapphire sheath fitted over the thermocouple. The apparatus may also comprise a thermowell, with the sapphire envelope being provided by the thermowell.
In accordance with another aspect of the invention, an improved apparatus for measuring the temperature in a gasification process comprising a thermowell and one or more thermocouples is provided. The improvement comprises a sapphire envelope for enclosing at least a portion of at least one thermocouple. The sapphire envelope may be in the form of a sapphire sheath fitted over the thermocouple. The thermowell may contain at least one barrier layer comprised of sapphire, with the sapphire envelope being equivalent to the barrier layer comprised of sapphire.